
with \(n^2=\varepsilon \) .
The first term in the above expression is the one usually detected in radiation experiments in optics. As in such cases \(n>1\) , it means that radiation forces acting in inhomogeneous regions in the fluid act toward the optically thinner region. This kind of experiments, started by Ashskin and Dziedzic back in 1973 [1]}, has later been repeated under different circumstances, and are showing the effect very clearly. Some recent papers can be consulted in Refs. [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]}. This kind of force is evidently of importance, both from a fundamental and a practical viewpoint, especially in microfluidic contexts.
